The present invention relates to coaxial cables for the high-frequency sector and to the production of polytetrafluoroethylene (PTFE) foam, in particular as cable insulation, a dielectric or a molded gasket.
Cable insulation should have the lowest possible dielectric constant xcex5r and the lowest possible dissipation factor tan xcex4.
These requirements are particularly well complied with by PTFE, ideally pure PTFE not modified with comonomers or with long unbranched chains, because contamination with end group branching or comonomers add additional dipoles. The dielectric constant of unsintered PTFE paste material is generally about 1.7 and that of sintered PTFE is generally between 2.05 and 2.1. Another very significant advantage of PTFE cable insulation is that, unlike polyvinyl chloride, polyethylene or polypropylene, it is incombustible.
Cable production generally proceeds by paste extrusion as in xe2x80x9cVerarbeitunigsbroschxc3xcre PTFE, (copyright)Hostaflon Kunststoffe der Hoechst AG [PTFE processing brochure: (copyright)Hostaflon Plastics from Hoechst AG], December 1986xe2x80x9d. However, this paste is not processed thermoplastically, rather its ability to be deformed is increased through the addition of lubricants. For further improvement in electrical properties, JP-B 42-13560 and U.S. Pat. No. 4,187,390 propose that paste extrusion be followed by a stretching process. The oriented tapes have a structure which permits the production of aerated cable sheathing, their density consequently being lower than that of conventional PTFE. There is a corresponding improvement in electrical properties. However, four steps are then required in the process, namely extrudate production, extrudate calendering tape stretching and winding the tape around a wire. This process is therefore costly and time-consuming.
To reduce the density further, JP-B 57-30059 proposes adding up to 15% of foaming agents. The plastic (PTFE) foamed in that publication is not thermoplastically processible. It is usually only thermoplastically processible materials which are physically or chemically foamed. With an additional foaming procedure, the process is still more complicated.
EP-A-808 865 teaches that besides thermoplastic processible fluoropolymer, PTFE can also be foamed with super critical CO2. However, in this process the fluoropolymer is first formed into the desired shape and only subsequently foamed under sinter conditions.
JP-A 11-124458 moreover proposes foams of suspension product. These have to be peeled from a block before they can be wound up as a tape. Again, this is a complicated process.
WO-A-91/12123 provides a simplified process. The PTFE paste is extruded onto wire and then oriented. However, a disadvantage is that the cable insulation does not adhere to the wire, and the insulation becomes deformed due to lack of dimensional stability when exposed to small mechanical loads, for example if it is handled without sufficient care. Although the dimensional stability can be increased by sintering this leads to other disadvantages, such as compromising the electrical properties, shrinkage losses and elasticity losses.
It is an object of the present invention to provide a simple process in which the porous PTFE is processed to give coaxial cables with excellent electrical properties.
According to the invention, this object is achieved by extruding a not-melt processible fluoropolymer, in particular PTFE together with a space holder, in particular a foaming agent, such that voids remain subsequent to the removal of the space holder. The space holder and in particular the foaming agent is used in an amount from 20 to 100 parts by weight of space holder or foaming agent for each 100 parts by weight of PTFE.